A typical exhaust purifying apparatus for an internal combustion engine, such as a diesel engine, has a collector in an exhaust passage for collecting particulate matter (“PM”) contained in exhaust gas, and performs regeneration control in which collected particulate matter is burned and removed for regenerating the collector. In the regeneration control, the amount of particulate matter collected by the collector (collection amount) is estimated according to the operating condition of the engine. When predetermined regeneration requirements are satisfied, which include the estimated collection amount being no less than a predetermined value, a fuel adding valve, which is provided separately from a fuel injection valve for driving the engine, adds fuel to exhaust gas in a section upstream of the collector. The added fuel is burned at the collector and generates heat. The generated heat increases the temperature of the collector to a temperature that removes particulate matter (about 600° C.). Accordingly, particulate matter is burned and removed so that the collector is regenerated.
For example, Japanese Laid-Open Patent Publication No. 2002-227688 discloses a technology related to such an exhaust purifying apparatus. According to the publication, an exhaust purifying catalyst, such as oxidation catalyst, is provided upstream of a collector. The exhaust air-fuel ratio is repeatedly made rich and lean at an appropriate interval, such that particulate matter is burned and removed.
Other than Japanese Laid-Open Patent Publication No. 2002-227688, Japanese Laid-Open Patent Publications No. 2002-332822 and No. 2003-20930 are also prior art documents related to the present invention.
Unburned fuel tends to be collected on an upstream end of an exhaust purifying catalyst. It is difficult to completely burn the collected unburned fuel, and unburned fuel remains as a deposit. The reason for this is as follows. The temperature distribution of the exhaust purifying catalyst and collector is that the temperature is low at the upstream end of the catalyst and increases toward the downstream end. Therefore, from the point of view that the temperature of the collector should be prevented from exceeding an upper limit value of a temperature range where the collector is capable of collecting particulate matter, the temperature of the exhaust purifying catalyst cannot be increased beyond a certain limitation. The temperature can be increased to a level to burn added fuel, but cannot be increased to a level to burn deposits. Therefore, although added fuel is burned, deposits remain without being burned.
The remaining deposits degrade the reactivity of the exhaust purifying catalyst, and causes particulate matter to remain unburned in the collector. As a result, when the exhaust air-fuel ratio is richened by adding fuel to exhaust gas during the regeneration of the collector, the remaining unburned particulate matter in the collector is burned rapidly. This can excessively increase the temperature of the collector. This problem is also present in a case where an exhaust purifying catalyst is not provided upstream of a collector.